review of 20 years of sediver silicone coated insulators ......iec 60815 standards, - and an...

SEDIVER RESEARCH CENTER, 46 avenue de Thiers, 03270 SAINT-YORRE, FRANCE

Review of 20 Years of Sediver Silicone Coated Insulators in the Field

J-M. GEORGE – Dr S. PRAT – F. VIRLOGEUX

SEDIVER RESEARCH CENTER

FRANCE

Abstract

Silicone coatings have been largely used in substations for at least 30 years. Likewise it has been used by

numerous utilities as a palliative approach to contamination problems for overhead line insulators.

Applications of the silicone coating can be performed in several ways: directly on insulator string in the tower,

on site before assembly in tower or in insulator factory by the supplier before on site delivery.

Current trends demonstrate that silicone coated toughened glass insulators are now being selected from the

design stage rather than as a fix of an existing pollution problem. This market is currently expanding including

in projects where polymers are no longer the preferred choice. This paper presents projects where silicone

coated toughened glass insulators have been successfully used for years in a variety of environments and

describes the benefits of this technology through service experience and follow up analyses.

Introduction

Among the various options to fight contamination problems on overhead lines the choice today lies between

using higher creepage distances on the insulators, and using silicone rubber housing materials or both. The

latter solution can be achieved through a silicone housing as it is the case for a composite insulator or by the

application of a silicone coating over a ceramic (glass or porcelain) insulator.

The failures encountered by quite a number of utilities with composite insulators have considerably impacted

the confidence in the technology while actually often used to solve contamination problems.

The ageing mechanisms reported in the last decade are also an indication of the impact of harsh conditions on

a component specifically installed there to sustain these very severe conditions.

More than 2 million of silicone coated toughened glass insulators (Sedicoat®) are in use today in more than

17 countries. They are systematically installed in harsh conditions where otherwise composite insulators would

be installed. The limitation for the use of composite insulators is related to their short lifetime and difficulties

of maintenance in service conditions where the severity of the environmental conditions is proven. The main

driver for the decision of utilities to engage into a coating solution instead of a composite insulator is long term

reliability, capability of live line work and easy inspection methods. All these features represent a major

attribute of toughened glass insulators especially when compared to composite insulators or even porcelain

discs.

Examples of service experience and performance of silicone coated toughened glass insulators are presented

in this paper covering a variety of conditions.

1. Successful ongoing experiences in service

Many papers published by Sediver over the years [1-4] relates historical development performed with TERNA,

the Italian TSO, who widely implemented the use of Coated toughened glass insulators in high voltage

overhead transmission line. After 11 years of large scale use in heavy polluted environment, the silicone

coating over toughened glass insulators technology keep offering reliable performance in service without any

reported electrical fault or need for washing in areas which were previously regularly subjected to maintenance

due to heavy pollution accumulation.

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Another long lasting experience is in Qatar on a 132 kV overhead Line Al Shala- Umm Bad. Silicone coated

toughened glass insulators with an open or aerodynamic profile were first installed in 1997 and are still in use

with very good performances. Some samples units were evaluated in 2015 and the benefit of the silicone

coating has easily been demonstrated after 18 years in service [1.] without any need for washing. The

environment of the line is characterized by a mix of desert, marine and industrial pollution sources.

Sediver also already reported many examples of projects in coastal environment in the USA, in Canada, in Sri

Lanka, North Africa, Latin America and many projects around the Mediterranean sea. All customers’

feedbacks report good results and satisfactory performances to mitigate pollution born issue.

This papers will focus on some special cases of applications which can offer some deeper understanding of the

benefits of the technology.

2. Experience in Peru, in highly polluted environment

Peru is an extremely biodiverse country with habitats ranging from the arid plains of the Pacific coastal region

in the west to the peaks of the Andes mountains vertically extending from the north to the southeast of the

country to the tropical Amazon Basin rainforest in the east with the Amazon river. The high voltage Peruvian

network is mainly based on 220 and 500 kV overhead lines. The power network developed in the late 2010’s

is mainly located on the western region of the country along the Pacific coast with a 500 kV Overhead line

running from north to south of the country. The environment of this line is characterized as desert, mountainous

environment with locally severe exposure to sea born pollution. This combination of contaminants make the

insulation of the line a critical point.

Following many line drops with composite insulators [5.] in a similar environment, the technology of

toughened glass insulator and often silicone coated toughened glass insulators was selected for many projects

and applications. Sedicoat® silicone coated insulators were used instead of regular toughened glass insulators

when the distance to the sea coast was less than 10 km.

Insulation design was based on IEC 60815-1, using a specific creepage distance of 31 mm/ kV or recently

renamed as USCD (Unified Specific Creepage Distance) of 53.7 mm/ kV phase to ground.

A typical design of the insulator string is based on 29 units of insulators with a 160 kN rating, fog profile with

a creepage distance of about 620 mm per unit. The first case to be studied in this paper is the overhead line

500kV Chilca – Carabayllo was erected in 2010 and energized in 2011.

The line route is described in Figure 1.

https://en.wikipedia.org/wiki/Biodiversityhttps://en.wikipedia.org/wiki/Andes_mountainshttps://en.wikipedia.org/wiki/Amazon_Basinhttps://en.wikipedia.org/wiki/Amazon_river

Figure 1 : Line route of the Chilca-Carabayllo 500 kV AC.

After about 5 years of service, a string of a suspension tower located in the south district of Lima was taken

down and samples were submitted to evaluation. A series of tests was performed as follow :

- visual inspection- the measure of the pollution level on the surface of these insulators according to IEC 60507 and

IEC 60815 standards,

- and an electrical test in clean fog chamber according to IEC 60507 standard § 6.7.

2.1/ visual inspection

First observations: all insulators display a layer of pollution built up on their surface (especially on bottom

surfaces) (Figure 2).

Figure 2: Example of pollution deposit on the bottom part of insulators.

Electrical damages due to the electrical activity linked to the accumulation of pollution have been observed

around the pins of the units removed from the line.

First type of damage is an attack of the zinc sleeve (sacrificial anode) as shown in Figure 3. This type of

degradation is the results of electrical activity around the insulator pin and is expected in such a polluted

environment. This is also a proof that the sacrificial anode is perfectly doing its job as no degradation could be

observed on the pin shanck. The pin sleeve protects the pin from corrosion and therefore ensures the long term

mechanical reliability of the insulator.

Figure 3: Insulator number 1(left) and Insulator number 2 (right)

Second type of damage is the degradation of the silicone coating localized on the anti-corona rib and classified

as light according to the SEDIVER Coating Erosion Class available in annex 1 (Figure 4).

Figure 4 : n°1 (high voltage end) classified as CE 1 / CE 2(left) and n°2 classified as CE 1 / CE 2 (right)

All others insulators in the string show an erosion level of the coating that is classified as CE1.

The section around the pin of the insulator will be the most severely challenged during the life of the insulator,

and hydrophobicity will decrease primarily there as shown in Figure 5.

Figure 5: Electric field around the pin of a suspension cap and pin insulator.

2.2/ Evaluation of the site pollution severity

The measures have been performed according to IEC 60507 appendix D6 and the pollution level is given

according to IEC 60815.

Insulator Time in service ESDD

(mg/cm²) NSDD

(mg/cm²) Pollution level

Insulator n°1 (Top surface)

About 5 years

0.17 3.10 Very Heavy

Insulator n°1 (Bottom surface) 0.41 12.5 Very Heavy

Insulator n°1 total 0.33 9.24 Very Heavy

Insulator n°14 (Top surface) 0.02 5.54 Heavy



Insulator n°19 (Top surface) 0.03 6.71 Heavy



Insulator n°29 (Top surface) 0.02 1.39 Medium



Average value for the string 0.20 11.43 Very Heavy

Figure 6 : graph of the pollution site severity

First it should be highlighted that from the 8 measurements, 6 results fall out of the limit of the graph given in

IEC TS 60815-1 (2008) and that in order to plot those values in the graph the limits for the highest NSDD values

have been increased from 4 to 20 g/cm². This clearly shows the unusually high values of the NSDD components

measured on those insulators.

A higher level of pollution can be observed on the bottom surfaces of the insulators compared to the top surfaces.

The lower level of pollution measured on top surfaces compared to the bottom surfaces might be the result of

the natural washing effect due to wind, rain or fog condensation.

Limit of the

graph in the IEC

document

A second comment is that the level of pollution along the string length is homogeneous, specially for the bottom

surfaces of the insulators. Regarding top surfaces, a difference can be noticed between insulator 1 (at the bottom

of the string on the high voltage end) and insulator 29 (at the top of the string near the tower structure) and can

be explained probably by the efficiency of the natural washing effect (no protection from any upper unit).

Pollution level on top surfaces is measured as “Heavy” according to IEC 60815-1 (2008) whereas the bottom

surfaces exhibit a pollution level that is classified as “Very Heavy” especially for the Non Soluble Deposit

Density component which is almost outside of the IEC 60815 chart limits (Figure 6). Those high NSDD values

can be explained by the dry and desertic direct environment of the tower.

Another commonly used factor to describe the site pollution level is the CUR factor which stands for

Contamination Uniformity Ratio. This factor is defined as the ratio of the ESDD component of the bottom

surface divided by the ESDD of the top surface.

Insulator 1: CUR = 2.4

Insulators 14, 19 and 29: CUR is about 10

Those CUR values clearly indicate that the pollution deposit is concentrated on the bottom surface of the

insulators.

Based on those measurements, it clearly appears that each tested insulator display a pollution level (top and

bottom surfaces included) that is ranked as “Very Heavy”.

2.3/ Clean fog voltage withstand test Short strings of 5 units (naturally polluted in service) have been tested in a clean fog environment according to

IEC 60507 § 6.7 in order to assess the electrical performance of the naturally polluted string of insulator and the

efficiency of the coated glass insulators (Figure 7).

The steam input rate is defined by IEC 60507 § 6.7 as 0.05 kg/h ± 0.01 kg/h/m3 of the test chamber volume.

The test consists in applying a voltage for 100 min according to test procedure B (§ 6.7.3 in IEC 60507).

The test is passed if no flashover occurs during the 100 minutes.

Test Short string description

Applied Voltage

Stress (phase to ground)

Stress (phase to phase)

Test Results

1 5 units 75 kV 41.3 mm/kV 23.9 mm/kV Withstand 100 min

2 5 units 95 kV 32.6 mm/kV 18.8 mm/kV Withstand 100 min

3 5 units 115 kV 27 mm/kV 15.5 mm/kV Withstand 100 min

4 5 units 135 kV 23 mm/kV 13.2 mm/kV Flashover 97 min

line 29 units 550 / √3 kV 56.3 mm/kV 32.5 mm/kV

Figure 7: test set up for the clean fog test

A short string of 5 units naturally polluted in service can withstand a voltage of 115 kV.

This voltage would correspond to a design of 27 mm/kV phase to ground (equivalent to 15.5 mm/kV phase to

phase) whereas the actual service design is about 32.5 mm/kV phase to ground if we consider a maximum

voltage of 550 kV.

In addition, during the test, leakage currents were recorded and the test at 115 kV only displays leakage current

with a peak intensity of about 20 mA during the 100 minutes of test. Those low values of leakage current are a

proof of the effectiveness of the silicone coated string design for the actual level of pollution after 5 years in

service.

3. Performance of degraded silicone coated insulators units

A second case of applications of Sedicoat® silicone coated toughened glass insulators in another 500 kV

overhead transmission line in Peru delivered interesting data regarding the performance of the units under a

degraded status linked to very challenging operating conditions as detailed here after.

The section of the 500 kV transmission line is in the south district of Lima, named Chilca District, where a

power plant is built on the pacific coast. Maintenance team of the local utility reported evidence of electrical

activity (dry band arcing activity but no flashover) on the first towers exiting the power plant, few meters away

from the ocean. (Figure 8)

Figure 8: site description for the Chilca Fenix overhead line

In this line section, the insulators strings were designed with a specific creepage distance of 31 mm / kV (phase

to phase) equivalent to a USCD of 53.7 mm /kV (phase to ground), using 28 units of an antifog insulator, with

a 160 kN rating.The line was first energised in 2013. A jumper string of tower 2 and a suspension string of

tower 3 were taken down for evaluation.

3.1/ visual inspection

Electrical activities leading to coating degradations can be observed all around the pin on the bottom side of the

insulator. Those degradations can be classified according to the Sediver coating erosion chart (see Annex 1).

Figure 9 shows the coating degradation all along the string of tower 2 and Figure 10 shows units from tower 3.

n°1 classified as CE 4 n°6 classified as CE 5



Figure 9: Close views on the insulators pins for Tower 2


Figure 10: Close views on the insulators pins for Tower 3

Evaluation of the hydrophobicity along the string of Tower 3 the day after string removal :

Insulator Hydrophobicity Class

on top surface

Hydrophobicity Class

on bottom surface

T.3- 1 (high voltage side) HC 3 and 6 cap edge HC 3/4

T.3- 5 HC 6 HC 5

T.3- 9 HC 6 HC 1/ 2

T.3- 13 HC 4 HC 5

T.3- 17 HC 5/6 HC 3/4

T.3- 21 HC 4/5 HC 2/3

T.3- 25 (towards ground side) HC 5/6 HC 2/ 3

It should be noted that in the very close vicinity of the insulator pin (few cm), in the area where some surface

degradations could be observed, the coating offers hydrophilic surface properties. Moving away from the pin,

the silicone coating gradually exhibits better hydrophobic properties, showing the influence that the electrical

field density may have on the hydrophobicity property of the silicone material. In the table above, the

hydrophobicity class given for the insulator is to be considered as the average hydrophobicity of the section,

even if a difference of performances can be observed from one location to another. The overall conclusion is

that despite a high level of pollution, evidence of electrical activity leading to a reduction of the hydrophobicity

properties, the string of Sedicoat® insulators can still maintain its insulation fonction. In addition it can be

mentioned that such surface degradations are not compromising the insulator integrity as the underneath

toughened glass is immune and mechanically safe. A similar situation on a composite insulator would require

a deep inspection of the units and units with eroded housing should be changed immediately to prevent

catastrophic failures of the line.

3.2/ Evaluation of the site pollution severity

The pollution level has been determined with the ESDD and NSDD method.

The measures have been performed according to IEC 60507 appendix D6 and the pollution level is given

according to IEC 60815.

Insulator Time in service ESDD

(mg/cm²) NSDD

(mg/cm²) Pollution level

Fenix T.3 n°1 (Top surface)

About 4 years (no washing)

0.4669 1.969 Very Heavy

Fenix T.3 n°1 (Bottom surface) 0.9897 3.004 Very Heavy

Fenix T.3 n°1 total 0.7283 2.486 Very Heavy

Fenix T.3 n°13 (Top surface) 0.0989 0.720 Heavy


Fenix T.3 n°13 total 0.1624 1.347 Heavy

Fenix T.3 n°25 (Top surface) 0.0870 0.961 Heavy


Fenix T.3 n°25 total 0.1254 1.164 Heavy

Average value for the string 0.339 1.666 Very Heavy

The pollution measured along the string is heterogeneous with a pollution profile along the string. The insulators

on the live end (bottom of the string) have collected more pollution than insulators on the ground end side of the

string (at the top). In this case, the bottom surfaces collect more pollution than top surfaces.

Figure 11 : graph of the pollution site severity

It clearly appears on this chart (Figure 11) that the pollution is characterized by strong ESDD components

(obviously related to the vicinity of the Pacific coast).

3.3/ Clean fog voltage withstand test

Short strings of 4 units (naturally polluted in service) have been tested in a clean fog environment according to

IEC 60507 § 6.7 in order to assess the electrical performance of the naturally polluted string of insulator and the

efficiency of the coated glass insulator.

The steam input rate is defined by IEC 60507 § 6.7 as 0.05 kg/h ± 0.01 kg/h/m3 of the test chamber volume.

The test consists in applying a voltage for 100 min according to test procedure B (§ 6.7.3 in IEC 60507).

The test is passed if no flashover occurs during the 100 minutes.

The tests results are :

Test Short string

description

Applied

Voltage

Stress

(phase to ground)

Stress

(phase to phase)

Test Results

Tower 2-1 2-3-4-5 50 kV 49.6 mm/kV 28.6 mm/kV Withstand 100 min



Tower 3 -2 6-7-9-17 70 kV 35.4 mm/kV 20.5 mm/kV Withstand 100 min

line 28 units 550 kV 54.7mm/kV 31.6 mm/kV

A short string of 4 units naturally polluted in service can withstand a voltage of 70 kV.

This voltage would correspond to a design of 35.4 mm/kV phase to ground (equivalent to 20.5 mm/kV phase

to phase) whereas the actual service design is about 54.7 mm/kV phase to ground if we consider a maximum

voltage of 550 kV.

The degradations of the silicone coating and the reduction of the hydrophobicity around the pins of several

units are not compromising the performances of the string.

4. DC case in China As a proof of the development of the silicone coating technology and of the fact that this technology is now

fully introduced in the market, the case of the HVDC line design by SGCC (China northern utility) should be

mentioned. For the last HVDC 800 kV projects, coated cap and pin insulators were specified for the tension

strings.

The Inner Mongolia XiMeng – JiangSu TaiZhou +/- 800kV ultra high voltage DC transmission line was built

with a 550 kN rating insulators for tension strings. Unlike previous cases in China, the specification required

that the insulators were factory precoated and no longer coated after installation in order to benefit from all the

advantages of an industrially controlled process rather than field hand made coatings. It is established today

that the longevity and perfomance is much better when the insulators are factory coated versus “on site”

treatment [7] The line erection work was completed early 2017 and required the supply of about 800.000 cap

and pin insulators.

For this project the customer requested to have 2 colours of coating (Red and White). As shown in the Figure

12, every 10 pieces a red coloured unit is placed in the string of white coated units in order to ease the inspection

of localization of any defects for the maintenance crew.

Figure 12: Tension strings of the Inner Mongolia 800kV HVDC line with red and white coloured coatings over toughened glass

insulators

5. Improved silicone coating properties Over the last 10 years of regular supply of silicone coated insulators around the globe to help mitigating

pollution born insulation issues, Sediver collected regularly the voice of the market through continuous

communication with end users. One of the major feedback received from the maintenance crew in the field is

that silicone coated insulator can easily be damaged during handling, storing, assembling. (Figure 13)

Such comments would obviously come primarily from utilities used to normal toughened glass. Basically a

very thin layer of rubber at the surface of a rigid material is more prone to degradation during on site

transportation, in case of impact, rough handling, etc... In addition it should be mentioned that silicone rubber

despite its numerous benefits for outdoor application is a relatively weak material in terms of mechanical

property.

Even if many test data can prove the fact that small defects (limited area to few cm²) will not adversily impact

the performance of the coating, the need was clearly expressed to work on this feature looking forward to be

able to offer a solution to the market which would overcome this weakness in terms of handling.

Figure 13: Example of aesthetical defect met on coated insulators

Sediver devoted resources in its R&D department to come up with a new silicone coating that would match

the following requirements when applied in factory:

- Basic criteria of the Sediver homologation such as a high hydrophobicity level to improve performance under pollution (key aspect of the product) but also including :

o 2000h multiple stresses ageing test (Figure 14) o Adherence to the glass surface

- Mechanical properties that would allow the shattering of the glass shell to not hide one of the key benefit of the toughened glass technology (fail-safe inspection through at a glance visual inspection)

- But mechanical properties that will allow an easy handling of the product with limited risk of degradation.

Newly developed Sedco® silicone coating can now match the above mentionned criteria and allow a more

user friendly handling, similar to what is more common for regular cap and pin unit handling (Figure 15).

Figure 14: Insulator coated with newly developed SEDCO® coating at the end of the 2000 h multiple stresses ageing test according

to TERNA specification LIN-00J116 Rev. 1 dated 08/05/2013.

Figure 15: Scratch resistance of the new coating

In addition, this Sedco® coating offers outstanding adhesion to the glass surface as shown through by the

boiling water test described in IEEE 1523 [5]. In this test silicone coated insulators are immersed in boiling

water for 100h and then the adhesion of the coating should be controlled. (Figure 16) This boiling water

immersion test can be used to check the adhesion of the coating to the glass shell.

Figure 16: Results of 100 h boiling water test for a commercially available and for the newly developed Sedco® coating

Conclusion

Sediver has acquired substantial R&D and field knowledge to offer a unique expertise on silicone coated

toughened glass solutions in substitution to traditional composite insulators for harsh environments.

The performance under severe or extreme pollution of silicone coated toughened glass insulators is proven by

numerous field results and the case of applications in 500 kV OHL in Peru is a typical example. The unique

combination of pollution performance (through the silicone coating), of the long term reliability of this product

(through the toughened glass insulator and factory applied coating process) and the at-a-glance inspection

capability make this product a perfect fit for highly demanding insulation strings.

This solution is more and more the preferred choice of utilities sensitive to full cost of ownership, reliability,

inspection and live line work.

Publications

1. George JM, Prat S, Virlogeux F. “Silicone coating on toughened glass insulator : Review of laboratory and field performance”, INMR World Congress, Munich, Germany, 18-21 October 2015.

2. George JM, Prat S, Virlogeux F. “Coating glass insulators for service in severe environments”, INMR, 2014, Vol. 22, No. 4, pp. 60-64.

3. Cherney EA, El-Hag A, Li S, Gorur RS, Meyer L, Marzinotto M, George JM, Ramirez I. “RTV Silicone Rubber Pre-coated Ceramic Insulators for Transmission Lines”, IEEE Transactions on

Dielectrics and Electrical Insulation, 2013, Vol. 20, No. 1, pp 237-244.

4. Rendina R, Guarniere MR, Posati A, George JM, Prat S, De Simone G. “First experience with factory-coated glass insulators on the Italian transmission network”, INMR World Congress, Rio de

Janeiro, 2007.

5. “South American utilities plan live line replacement program to solve problems with composite insulators”, INMR, 2011, Vol. 19, No. 1, pp. 82-89.

6. “IEEE Guide for the Application, Maintenance, and Evaluation of Room Temperature Vulcanizing (RTV) Silicone Rubber Coatings for Outdoor Ceramic Insulators” IEEE Standard 1523, 2002.

7. Znaidi R. “Maintenance Strategy Designed to Support Gulf Regional Energy Security”, INMR Weekly Technical review, 2017, No. 37, September 11.

Annex 1 : SEDIVER Coating Erosion Chart

review of 20 years of sediver silicone coated insulators ......iec 60815 standards, - and an...

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